A groundbreaking discovery by astronomers has unveiled a star system at the pivotal moment when solid grains began to form planets. This remarkable observation offers a glimpse into the initial stages of planetary formation, akin to how our own Earth was born. The infant star, designated HOPS-315, is located approximately 1,300 light-years away, equating to about 7.6 quadrillion miles, within the constellation Orion.
Surrounding HOPS-315 is a disc composed of gas and dust, where temperatures are sufficiently high to bake rocks but still cool enough for those rocks to re-form. This duality creates an ideal environment for the formation of solid materials that eventually coalesce into planets. The research, led by Melissa McClure from Leiden University, represents a significant advancement in our understanding of planetary assembly. The findings provide scientists with a unique “live laboratory” that mirrors the early stages of our own Solar System.
Evidence from primitive meteorites indicates that the Solar System’s formation clock began ticking 4.567 billion years ago, with tiny calcium-aluminum-rich inclusions serving as key markers. These inclusions condensed from a superheated vapor and played a crucial role in seeding every terrestrial planet we know today. Capturing this condensation step, particularly outside our own Solar System, has long been a challenge for astronomers.
The recent observations represent the first instance where a telescope has detected gas-phase silicon monoxide (SiO) alongside newly crystallizing silicates within the same region of a protoplanetary disc. The team identified these minerals in an orbit comparable to that of our asteroid belt, which is significant because it connects early chemistry to a region that later supplied Earth with essential water and metals.
The James Webb Space Telescope (JWST) played a crucial role in this discovery by capturing infrared light that penetrates the dust surrounding HOPS-315, revealing the distinct signature of hot SiO molecules. These molecules emit light at temperatures around 2,200°F, a condition that vaporizes most common rocks. Subsequently, the Atacama Large Millimeter/submillimeter Array (ALMA) measured the same area at millimeter wavelengths to map the source of the glow. By combining these observations, scientists confirmed that both gas and solid forms of silicon reside within a ring no more than 2 astronomical units from the star.
Disentangling these observations was complex due to HOPS-315’s powerful jet, which is rich in SiO. The team utilized velocity checks to differentiate the jet’s signal from that of the disc, noting that the jet gas moves outward rapidly while disc material orbits more slowly. Further validation involved comparing the brightness of various SiO lines, which aligned with laboratory predictions for vapor that is actively condensing, adding a layer of confidence to their findings.
The presence of crystalline silicates indicates that cooling vapor is meeting a sharp temperature drop, leading to ongoing condensation. “This process has never been seen before in a protoplanetary disc or anywhere outside our Solar System,” stated Edwin Bergin from the University of Michigan, a co-author of the study. He emphasized that the minerals observed are identical to those found in 4.5-billion-year-old meteorites on Earth, highlighting their significance in understanding planet formation.
Co-author Logan Francis from the Advanced Functional Fabrics of America (AFFOA) noted that the condensation zone is located at nearly the same orbital radius as our own asteroid belt, suggesting a direct link between the newly forming minerals and existing asteroids. These mineral grains, although measuring less than a micrometer across, represent the initial steps toward the creation of larger planetesimals. Through electrostatic forces, these grains will clump together for thousands of years until gravitational forces take over.
Laboratory studies indicate that minerals rich in silicon and oxygen condense first, followed by iron-nickel alloys and other volatile compounds. The detection of hot SiO vapor around HOPS-315 suggests that a similar sequence of chemical events is occurring there. By estimating the star’s luminosity and the temperature gradient of the disc, McClure’s team estimates that crystalline silicates could have a mass equivalent to about one-tenth of the Moon, which is ample material to seed multiple rocky planets if subsequent growth is effective.
Isotopic analyses of chondrules suggest that the earliest building blocks of our own Solar System formed within the first million years. The ongoing observations of HOPS-315 now allow researchers to test this timescale in real-time. Correlating astronomical data with isotopic chronometers promises a clearer understanding of planet formation, moving beyond the limitations of meteoritic studies.
In the coming year, ALMA will revisit HOPS-315 to search for water ice located further out in the disc. Should water be found beyond the silicate ring, astronomers can investigate whether rocky seeds migrate inward before acquiring ice mantles, a crucial step in explaining how Earth came to possess its oceans. Meanwhile, JWST will monitor the evolution of the SiO signature—an ongoing decline would suggest vapor freezing out, while a surge could indicate heating bursts from magnetic flares or spiral shocks.
Beyond the specific observations of HOPS-315, this discovery enhances confidence in the notion that rocky planets are common throughout the universe. Early condensation may initiate planet formation well before gas discs disintegrate, allowing for the migration, collision, and stabilization of newly formed worlds into their orbits. Elizabeth Humphreys, an astronomer at ESO who did not participate in the study, expressed her admiration for the team’s ability to identify the initial solids, highlighting how the combined capabilities of JWST and ALMA are revealing a universe where the steps toward life-bearing planets commence earlier than previously anticipated.
The groundbreaking study has been published in the esteemed journal Nature.
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